Pulse oximetry involves the continuous, noninvasive monitoring of the oxygen saturation level in blood perfused tissue to provide an early indication of impending shock. An oximeter probe (sensor) is secured to the patient and provides an electrical signal to an oximeter "box." The box houses electronic circuitry to process this electrical signal and generate human-readable indicia of the patient's blood oxygen saturation level. Both disposable and non-disposable sensor probes for this purpose are widely used.
Current disposable probes typically comprise a flexible substrate (e.g., foam, fabric) having a light emitting diode (LED) and a photosensor spaced apart from one another and secured to the substrate. The substrate is adhesively attached to a patient's skin, preferably on the finger, nose, or ear of an adult, or on the foot of an infant. When the sensor is secured to the patient, the tissue is disposed between the LED and the photosensor such that light emitted by the LED passes through the tissue and is received by the photosensor.
Changes in the amount of light absorbed by the photosensor are caused by changes in the optical absorption of certain wavelengths by the blood-perfused tissue. The absorption characteristics of the transilluminated tissue are related to the oxygen saturation level of hemoglobin flowing through the tissue. These variations in light absorption caused by changes in oxygen saturation permit the direct, noninvasive monitoring of arterial oxygen content.
These and other similar medical devices are well known. See, for example, Smart et al. U.S. Pat. No. 3,769,974, which relates to a photo-optical blood pulse measurement transducer, and Pinder U.S. Pat. No. 4,091,823, which relates to a transducer for monitoring heart rate.
A variety of support structures have been devised for adhering probes and electrodes to skin surfaces. See, for example, Striese U.S. Pat. No. 4,350,165, and Gordy U.S. Pat. No. 3,599,629, which discloses a disposable biopotential skin electrode comprising a deformable, synthetic polymeric material having an adhesive coating.
Goodman et al. U.S. Pat. No. 4,830,014 describes a sensor for measuring arterial oxygen saturation using noninvasive photoelectric techniques. In a preferred embodiment, the sensor comprises a flexible, planar substrate having an LED mounted near a first end thereof and a photosensor spaced apart from the LED and mounted in a second end thereof. The sensor further includes an adhesive backing to facilitate close conformance to a patient's fingertip, such that the blood-perfused tissue lying between the LED and the photosensor is transilluminated by the light from the LED. Beginning at column 1, line 44, Goodman states that a common problem with existing oximeter sensors arises from their incompatibility with a patient's anatomy. More particularly, the physical construction of the sensors renders them bulky and difficult to securely fasten to a patient's appendage (e.g., finger, foot, nose, ear), resulting in differential motion between the patient and the sensor during patient movement. This relative motion, in turn, causes signal distortion (motion artifact.)
Prior art attempts to eliminate motion artifact often produced undesirable occluding effects due to, for example, the spring pressure applied by clip-like devices, resulting in insufficient pulse amplitude to reliably measure blood flow. Goodman proposed solving this problem by integrating the light source and photosensor into the adhesive fastener. The present invention improves further upon the Goodman probe by substantially changing the way in which the probe is mounted to an extremity and the way it maintains alignment between the LED and photosensor.